A histogram (or empirical cumulative distribution) is a graphical way of representing a distribution of data samples. In histograms, the frequency may be a percentage of the total number of samples such that the total area of the rectangles is normalized to 1.
Some sets of data samples may be very large, and may even be continuously collected, such as data samples being generated by an online survey that is on-going. The number of samples may not be known, making it difficult or resource consuming to construct a histogram using all the samples.